The present invention concerns shape memory actuators of the type including:
a shape memory actuator element connecting a support structure to an actuated member, for the purposes of moving the latter from a first position to a second position when the shape memory actuator element is heated above a predetermined temperature, and
return spring means biassing the actuated member towards its first position.
Shape memory actuators have already found a variety of applications in different technological fields. These actuators make use of actuator elements composed of a shape memory material, typically a nickel and titanium alloy, which possesses the property of contracting when its temperature exceeds a certain transition value. The Applicant has already presented various patent applications regarding shape memory actuator applications, such as for the adjustment of motor vehicle rear-view mirrors, deflector plates in air conditioners for motor vehicles, and various other devices. The advantage of these actuators resides in the fact that they have an extremely simply structure, are light and occupy little space, whilst simultaneously providing reliable and efficient operation.
A possible drawback that limits the application of these actuators resides in the fact that they exhibit an intrinsically monostable mode of operation. In other words, they can be switched between a rest condition and an active condition by heating the element made of a shape memory material, this heating being achieved by passing an electrical current through the actuator element for example. When the actuator element cools down, when current is no longer present for example, it automatically returns to its rest condition. This return motion is often aided by providing return spring means, as already defined above. Consequently, with these actuators, if it is wished to keep the actuated member in the position corresponding to the actuator""s active position, the actuator must be kept hot, i.e. by continuing to pass an electrical current through it, for example. Alternatively, it is possible to provide a mechanical fastening system for maintaining the actuator in position even after it has cooled down, but in this case, it is also necessary to provide an additional actuator device for releasing the fastener mechanism. Another known solution is that of providing two opposing actuators that are selectively activated to maintain the actuated member in a first or second position, even when the other actuator is xe2x80x9ccoldxe2x80x9d. All of the above solutions are complicated, bulky and expensive.
The object of this invention it that of realizing a shape memory actuator that exhibits bi-stable operation, in other words one that is capable of remaining stable in both the rest position and the active position when the actuator element is xe2x80x9ccoldxe2x80x9d, and that provides a single shape memory element for this purpose, thereby advantaging compactness, costs and constructional simplicity.
In order to achieve this objective, the subject of the invention is a shape memory actuator possessing the characteristics indicated at the beginning of this description and which is also characterized in that suitable holding means are associated with the said actuator element for holding the actuated member in its second position even when the shape memory actuator element cools down, said holding means being deactivated to allow the controlled organ to return to its first position following a further activation of the shape memory actuator.
In a practical form of embodiment of the invention, the said holding means include a cam guide mounted on the support structure and a cursor that can move along the guide and is operatively connected to the actuated member, said cam guide having a first dead point corresponding to the first position of the actuated member and a second dead point, corresponding to the second position of the actuated member, in which the said cursor is restrained due to the return effect of the said spring means.
When the actuated member is in the first position, heating of the actuator element causes the actuated member to move to its second position. Simultaneously, the said cursor moves along the cam guide until it reaches the said second dead point, where it is blocked by the cam guide, even when the shape memory actuator is cooled down. To bring the actuated member back to its first position, it is necessary to activate the actuator element a second time, in order to make the cursor leave the second dead point and then allow it to return to the start position under the effect of the return spring means and with the actuator element cooled down. In other words, each successive heating of the actuator element provokes either the passage of the actuated member from its first position to its second position, or the passage of the actuated member from its second position to its first position, this actuated member always being restrained in the position that it reaches each time after the actuator element cools down.